. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE BLACK SEA 411 of thousands as one moves away from the coast, and especially in zones with strong influence of coastal run-off. The largest number of micro-organisms is found at depths of 10 to 75 m. Bacteria biomass changes with the same regularity. The minimum number of micro-organisms is found at depths of 75 to 150 m, where the bacterial population of the upper layer is replaced by the community of the hydrogen sulphide zone (mainly by a particular group of filamentary micro-organisms). The
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE BLACK SEA 411 of thousands as one moves away from the coast, and especially in zones with strong influence of coastal run-off. The largest number of micro-organisms is found at depths of 10 to 75 m. Bacteria biomass changes with the same regularity. The minimum number of micro-organisms is found at depths of 75 to 150 m, where the bacterial population of the upper layer is replaced by the community of the hydrogen sulphide zone (mainly by a particular group of filamentary micro-organisms). The number of micro-organisms in the hydrogen sulphide zone is considerably DISTANCE FROM THE COAST, MILES. Fig. 195. Distribution of micro-organism population in Black Sea and the alteration of its density with the distance from the coast (Kriss). higher than that of the surface oxygenated zone. Another sharp rise in the number of micro-organisms is observed in the upper layer of the sea-bed, where it reaches 1^ to 3 milliards per 1 g of the wet weight with a biomass of 3 to 6 g/mA The effect of river discharge on the number of micro-organisms in the water is clearly shown by M. Lebedeva (Fig. 196). The quantitative range of micro-organisms changes in winter, owing to a considerable fall in temperature: their main mass is then concentrated in the 0 to 50 m layer. The number of bacteria decreases sharply deeper down, only increasing again in the hydrogen sulphide zone (Fig. 197). Kriss gives comparative values of phyto- and zoo-plankton biomass as an illustration to his data {Table 163). According to these data the biomass of bacteria is considerably higher than the quantity of plant and animal plankton, and if we take into consideration a much greater biological activity of the micro-organisms, their immense importance will become evident both for the phytoplankton and zooplankton of the surface zone and for the biochemical processes in the hydrogen sulphide zone. In the oxygenate
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